Welcome to the Lightroom Community. I'll need more info to help you figure this out.
You should find the hidden sync menu when you press the OPT (macOS) or the ALT (Windows) key in the Lightroom Classic Preferences > Lightroom Sync tab.
Apply flags or rating to some of the images syncing in Lightroom classic & let Lightroom sit idle for 30-40 minutes to finish syncing. Log on to Lightroom Web (https://lightroom.adobe.com) & check for any sync issues on the web. Before making any changes to sync issues, do locate & back up these images away from Lightroom. Lightroom sync might not function as expected as there are sync issues.
Thanks Sean. I deleted all synced images in iCloud. And found and was able to delete the images which were blocking sync- owing to chnaged location of the images. Have now re-synced collections and all is good.